The pH of pigment based inks tends to drift over time. This is an ongoing problem. Typically pigments are dispersed using polymeric dispersants that carry charges on carboxylate groups. These charges repel each other, keeping the pigment particles stabilized in dispersion. This is called electrostatic stabilization. The other way pigments are stabilized is by steric stabilization, but the former is more important to stability of pigment dispersions, and hence preferred. However, the pH of such dispersions tends to drift over prolonged storage.
When the pH drop occurs, the carboxylate groups get protonated and the total charge needed to stabilize the pigment dispersion decreases. When this decreases enough, the pigment dispersions will flocculate and crash. This flocculation can cause nozzle clogging and potentially pen failure. The drop in pH can be accelerated by storing the ink at an elevated temperature (e.g., 70° C.).
Prior solutions have included making the ink at a higher pH than necessary to keep the pigment particles dispersed. This is expected to allow for the drop in pH. However compatibility of pen materials with a high pH ink can be a potential problem.
Another approach is for the end user to shake the ink cartridge before installing in the printer, while another approach is to reduce the labeled useful lifetime of the pigment based inks.
Another approach has been to add a large amount of dispersant polymer. This could lead to several problems, such as expense and increased viscosity, where the latter can cause problems with ink jet pen performance.
Accordingly, it would be advantageous to have a rational approach to polymer design for preparing stable ink dispersions.